Etching solutions and process for etching members therewith



United States Patent ETCHING SOLUTIONS AND PROCESS FOR ETCHING MEMBERS THEREWITH No Drawing. Application June 4, 1954, Serial No. 434,646

8 Claims. .(CI. 41-42) This invention relates to a novel etching solution and a process for treating the surface of metals and metalloids such as germanium and silicon for use in semiconductor devices such asrectifiers and transistors.

In such semiconductor devices, the most desirable characteristics. are obtained only if the germanium or silicon surface is given a suitable preparatory treatment. This treatment involves cutting a section, as for instance a wafer, from an ingot of germanium or silicon,,grinding the surface to a dull gray finish with a suitable wet abrasive powder, washing and drying the surface, and thenf etching the surface in a suitable etchant solution. j

The etchant solution used in this treatment has a considerable effect upon the characteristics of thejfinished device. Etching action is usually initiated at small surface imperfections such as pits or scratches, andi'thus the microscopic condition of the surface, which is impossible to reproduce exactly from one occasion to the next, plays an important role in the activity of the etching process. It is not understood exactly what chemical reactions are necessary for reproducible etching, but it is observed that etch activity will vary from one batch to another of supposedly identical etching solutions, thus adversely affecting reproducibility of surface conditions. '';With the discovery of the etching solution and process of our invention, however, it is possible to obtain more uniform and reliable results than heretofore possible. The etch of our invention imparts to the surface of wafers prepared from a single crystal, an etched surface which is fiat, smooth and clean. The etching solution of this invention is further useful in treating a seed crystal to produce clean, regular surfaces thereon so that it is possible to pull large single crystals of silicon from a melt free of lineage boundaries and having a minimum amount of dislocations.

An object of this invention is to provide an etching solution comprising as its essential ingredients an aqueous mixture of hydrofluoric acid, nitric acid, and dissolved mercury.

A further object of this invention is to provide a method for treating the surface of either germanium or silicon with a novel etching solution comprising as its essential ingredients an aqueous mixture of hydrofluoric acid, nitric acid and dissolved mercury.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

We have discovered an aqueous solution of hydrofluoric acid, nitric acid and mercury that may be used effectively for etching either germanium or silicon over a range of proportions and dilutions. We initially make up 100 parts by weight of a mixture of from 55% to 92% by weight of hydrofluoric and nitric acids and from 8% to 45% by weight ofwater. The ratio of the hydro; fiuoric to nitric acid varies from 9:1 to 1:12 by weight. We then add mercury to the solution, calculated as free mercury, in the proportions of from 0.01% to 2% of the 100 parts of the aqueous acid solution. It has been found that optimum results are obtained if the mercury is from 0.1% to 1.2% of the weight of the aqueous acid solution.

The mercury may be introduced into the solution in any suitable form which is soluble in and compatible with the acids. We have obtained good results by dissolving mercuric nitrate or mercuric chloride in an amount sufficient to provide the necessary range of mercury. Mercuric oxide and free mercury can be added and will be dissolved by the acids.

Convenient commercial sources for the ingredients of the etching solution, as received from a supplier, are

usually 48% to 50% hydrofluoric acid-by weight having a specific gravity of approximately 1.15; and concentrated nitric acid which contains 70% ,to 72% by weight fHNOa with a specific gravity of approximately 1.42. A suitable weighed amount of mercuric nitrate or mercuric chloride is dissolved in distilled water and, if necessary, a suflicient amount of distilled water is added 'tomake up the balance of the solution.

The length of time required for treatment of germanium or silicon bodies depends greatly upon the concentration of the acids in the etching solution; Good results have been obtained with as short an etching time as 5 seconds. The usual etching time in most cases :is about 1 minute at room temperature (25 0). While the etchingjtime 18 not particularly critical, care, however, should be taken not to extend the etching'unduly since an excessive amount of germanium or silicon may be removed and also the surface may be adversely affected.

- After the completion of the etching, the film the etchant on the surfaces of the etched members should be removed by rinsing or washing with a purified fluid free from significant impurities which might otherwise contaminate the treated surface. Suitable fluids are methyl alcohol, dilute nitric acid and double distilled water The surface is then dried in an-air blast or by other suitable non-contaminating means.

The following examples are illustrative of the practice of this invention.

Example I An etching solution was prepared by admixing 3 cc. HF (50% 3 cc. HNOa (70%) and 4 cc. of an agueous solution containing a total of 0.133 gram of Hg(NOa)2. A body of silicon with a suitably ground surface was etched in this solution for 60 seconds. There was a weight loss of 13.0 mg./cm. A bright, smooth .jetched surface with well defined crystal boundaries was produced.

Example II The procedure of Example I was repeated butusing 4 cc. HF (50%) and 2 cc. HNOa (70%) and the' same quantity of Hg(NO3)2 solution. There was a weight loss of 16.5 mg./cm.'- of silicon from a similar body. Equally good etching results were obtained.

Example III Example IV An etching solution was prepared by admixing 6 cc. HF (50%), 3 cc. HNOs (70%), Ice. H20 and 0.1333 gram of Hg(NO3)z. A body of silicon was etched in this sglution for 15 seconds. There was a weight loss of 21.7 mg./cm. A bright smooth etched surface was produced.

Example V An etching solution was prepared by admixing 8 cc. 4 cc. HNOa (70%) and 0.266 gram of HgClz dissolved in 8 cc. of distilled water. A body of silicon with ground surfaces was etched in this solution for 1 minute. There was a weight loss of 8.8 mg./cm'-. A bright smooth etched surface was produced.

Example VI An etching solution was prepared by admixing 37 cc. HF (50% 15 cc. HNOs (70%) and 2.0 grams of Hg(NO3)2 dissolved in ,48 cc. of distilled water. A body of germanium with a ground surface was etched in this solution for three minutes. There was a weight loss of 1.6 mg./cm. The etching action was relatively small as compared with the treatment of siliconflin Examples I through V, however, the etch gave an improvement in the germanium surface over that of a lapped germanium surface. A slight stain was imparted to the surface of the germanium by the etching solution of this Example VI, but the stain was removed by wiping, leaving a bright surface.

When the etching solution of this Example VI was used to etch a body of silicon, equally good results were obtained as illustrated in Examples I through V.

It is intended that all the matter contained in the above description shall be deemed to be illustrative and not limiting.

We claim as our invention:

1. In the method of treating a surface of a semiconducting material selected from the group consisting of silicon and germanium, the steps comprising etching said surface in aria-aqueous solution of hydrofluoric acid, nitric acid, and mercuric nitrate, and then rinsing said etching solution from said surface.

2. In the method of treating a surface of a semiconducting material selected from the group consisting of silicon and germanium, the steps comprising etching said surface in angaqueous solution of hydrofluoric acid, nitric acid, and mercuric chloride, and then rinsing said etching solution from said surface.

3. In the method of treating a surface of a semiconducting material selected from the group consisting of silicon and germanium, the steps comprising etching said surface in an aqueous solution comprising essentially 100 parts by weight of a mixture of from 55% to 92% by weight of acids and from 8% to 45 by weight of water, the acids comprising HF and HNOz, the ratio of HF to HNO: varying from 9:1 to 1:12 by weight, and mercury dissolved in the solution in an amount of from 0.01% to 2% ofithe 100 parts of the aqueous acid solution.

4. In the method of treating a surface of a semiconducting material selected from the group consisting of silicon and germanium, the steps comprising etching said surface in an aqueous solution comprising essentially 100 parts by weight of a mixture of from 55% to 92% by weight of acids and from 8% to by weight of water, the acids comprising HF and HNOs, the ratio of HF to HNO: varying from 9:1 to 1:12 by weight, and mercury dissolved in the solution in an amount of'from 0.01% to 1.2% of the 100 parts of the aqueous acid solution.

5. A solution particularly adapted for etching the surface of a-semiconducting material, comprising essentially 100 parts by weight of an aqueous acid mixture comprising from to 92% by weight of acids and from 8% to 45% by weight of water, the acids comprising HF and HNOa, the ratio of HF to HNO: varying from 9:1 to 1:12 by weight, and mercury dissolved in the solution in an amount of from 0.01% to 2% of the parts of the aqueous acid solution.

6. The solution of claim 5 in which the mercury is present in solution as mercuric nitrate.

7. The solution of claim 5 in which the mercury is present in solution as mercuric chloride.

8. A solution particularly adapted for etching the surface of a semiconducting material, comprising essentially 100 parts by weight of an aqueous acid mixture comprising from 55% to 92% by weight of acids and from 8% to 45% by weight of water, the acids comprising HF and HNQs, the ratio of HF to HNO: varying from 9:1 to 1:12 by weight, and mercury dissolved in the solution in an amount of from 0.1% to 1.2% of the 100 parts of the aqueous acid solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,600,997 Lark-Horovitz et al June 17, 1952 

1. IN THE METHOD OF TREATING A SURFACE OF A SEMICONDUCTING MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICON AND GERMANIUM, THE STEPS COMPRISING ETCHING SAID SURFACE IN AN AQUEOUS SOLUTION OF HYDROFLUORIC ACID, NITRIC ACID, AND MERCURIC NITRATE, AND THEN RINSING SAID ETCHING SOLUTION FROM SAID SURFACE. 